Tommy is the older brother of Declan and Ellie Walker. They traveled as a family hunting until his mother was killed by a werewolf about 5 years ago. Thanks to a small mistake made by Tommy during that hunt, their father has blamed him for their mother's death, and the guilt most likely still weighs on him pretty heavily. There were a lot of arguments between the two men until three years ago another hunt didn't quite go as well and Tommy was kicked out by their father.
Since then he has been on his own. At one point he made his way to their paternal grandfather's home and ended up taking possession of it. He came across some old records and has discovered that their great-grandfather was a member of an organization called the Men of Letters and that he and his siblings are now legacies, along with a few other families, including the Winchesters.
Tommy isn't the best at hunting but he is great at investigating and research.